The present invention relates to a polypropylene resin composition that contains a moldability improver to attain good moldability and prevent molding defects such as flow marks and voids for improvement in molding appearance. The present invention also relates to a molded part of the polypropylene resin composition for suitable use as an automotive exterior part and a production method thereof.
For the purposes of weight reduction and process flexibility, resin materials are widely used for automotive exterior parts etc. Although many of the resin molded parts are given painting or coating for industrial designs, some of the resin molded parts have their resin surfaces unpainted or uncoated in view of design and cost effects. It is thus desired that, for appearance improvements, the unpainted or uncoated surface portions of the resin molded parts sustain no or minimal molding defects such as so-called flow marks or tiger marks (wavy or rimple marks on the surfaces of the resin molded parts) and voids (spherical air voids in the thick and corner portions of the resin molded parts) due to improper flow of the resin materials into the molds.
Japanese Laid-Open Patent Publication No. 2003-253083 proposes a polypropylene resin composition for an automotive exterior part, which contains 100 parts by weight of propylene-ethylene block copolymer (A) having an intrinsic viscosity of 4 dl/g or higher as determined by extraction with xylene at 25° C. and 0.03 to 3 parts by weight of alkylbenzoate and hindered amine compounds (B). This proposed resin composition allows an improvement in the appearance of the resultant resin molded part.